This invention is in the field of development of images. More particularly, the invention relates to enhancing the optical density of developed images.
The general process known as ionography is a means for making X-ray images without the utilization of silver halide film. The basic process was disclosed by E. L. Criscuolo in NAVORD Report 4033 of July 6, 1955, in U.S. Pat. No. 2,900,515, in an article by R. A. Youshaw and J. A. Holloway in Nondestructive Testing, September - October, 1959, and by K. H. Reiss in Z. Angew. Physik, Vol.19, p. 1 (1965). This process comprises the utilization of two parallel plate electrodes having a gap therebetween. A d.c. voltage is applied across the electrodes such that one is a positive electrode and the other a negative one. If the positive electrode is nearest the X-ray or gamma ray source, it must not absorb much of this electromagnetic radiation. Typically the positive electrode has affixed to it an image-receiving sheet which may be optically transparent or opaque but must be an electrical insulator such as a thin sheet of a plastic film or the like. The negative electrode, if it is the electrode most distant from the radiation source, has a thin film or layer of a material which is an efficient absorber of X-rays applied to it. For example, in the aforementioned Reiss reference a heavy metal, such as lead, was disclosed as an absorber of the X-rays, and was in effect a photoemitter. The image-receiving insulator on the anode and the photoemitter layer on the cathode face each other across the gap between the electrodes with the object to be examined being disposed between the X-ray source and either the outer side of the anode or cathode, preferably on the outer side of the anode. A quenching gas is flowed, or in cases may be stationary, in the gap between the electrodes.
When the object is disposed adjacent the anode and is irradiated by X-rays or gamma rays, this electromagnetic radiation is differently absorbed by the object and passes through the transmissive anode and insulator layer affixed thereto and across the gap to strike the photoemitter which, as a consequence, ejects electrons having energies up to many kilo-electron volts. The number of electrons emitted from any area or portion of the photoemitter is dependent upon the number of X-ray photons absorbed in that portion, the depth of the absorption, and the photon energy. On leaving the photoemitter surface, the electrons find themselves in a d.c. field between the electrodes and travel toward the positive electrode. The quenching gas serves to slow down the electrons so that they will not scatter when reaching the insulator and to increase their number by secondary ionization. Upon arriving at the insulator surface, the electrons, and any negative ions which may have been formed by secondary ionization, are collected in an image configuration forming a latent electrostatic image consisting of negative charges corresponding to elements or portions of the object which are relatively transparent to X-rays and no charge or fewer charges corresponding to portions or elements of the object which are opaque or relatively opaque to X-rays. This latent image is then made visible by development or by cathode ray tube display techniques.
The development of the latent electrostatic images is accomplished generally by contacting the latent images with powder particles normally utilized for developing such images, not only in ionographic processes but in other processes where latent electrostatic images are formed, such as xerography and the like. Typical developer powder particles include charcoal, carbon black and various carbonaceous type pigments. Additionally, finely divided material, such as powdered resins having pigments or dyes added thereto, can be used. The use of such resins is desirable where the formed image is to be ultimately fused by heat or other means. The preferred particle size of the powder material is relatively small in order to maintain a good resolution of the developed image. For example, particles having average diameters of the order of 1 to 10 microns are normally utilized. The latent electrostatic image can either have a positive or negative charge, depending upon the polarity of the adjacent electrode utilized during the process of forming the image. Similarly, the powder particles have a charge thereon which can be induced by means of a corona discharge or in other ways. If the powder particles have a charge opposite to that of the latent image, they will then be attracted to the latent image charges forming a positive image. Alternatively, if the powder particles have a charge of the same polarity as that of the latent image, they will be repelled by the latent image and cover the substrate in the areas not occupied by the latent image, thus in effect forming a negative image. Hence the powder particles are given a charge, either opposite to or the same as the latent electrostatic charges on the substrate, depending upon the type of developed image desired.
The density of the developed image is basically affected by the amount of latent electrostatic charge produced on the substrate material. The density is also affected by the amount of developer powder utilized but there is a maximum density that can be achieved and this is determined principally by the strength of the electrostatic charge of the latent image. Thus, an excess of developer powder cannot make an image any denser than the limitations set by the latent image. On the other hand, a lesser amount of powder can decrease the density regardless of the amount of the latent electrostatic charges on the substrate. The ionographic process, which can be used to produce images of internal portions of the human body heretofore typically accomplished by conventional exposure of film by X-rays, is particularly useful in mammography. While normal X-ray procedure involves the production of a photographic image on film, in ionography a similar image is produced by development of electrostatic charges deposited in latent image configuration on an insulative substrate. In ionography as in conventional X-ray photography, there is a generally optimum preferred density for a developed image.
A particular advantage of ionography, as compared to normal X-ray techniques, is the ability to produce clear images with reduced X-ray exposures. But it is desirable that clear developed images of optimum density be produced with still lower X-ray exposures. As indicated above, the strength of the electrostatic charges that define or constitute the latent image affects the density of the resulting optical or visual image. But the strength or amount of charge does not affect to any great degree the resolution or other image characteristics. Hence, if optically denser images can be made with latent electrostatic images of a given field or level of charge, then excellent developed images can be produced at lower X-ray exposures.
As will be further appreciated from the discussion of the present invention, the process and product thereof will have applicability to processes in addition to ionography, such as xerography and the like wherein latent electrostatic images are produced on insulative substrates and subsequently developed. In such other processes, using the same exposure levels for production of latent electrostatic charges on the substrate, one can increase the optical density of the developed image in accordance with the present invention, or one can produce developed images of the same desirable level of optical density with lower levels of exposure. The ability of the present invention to substantially improve the optical density of images is of value in systems other than those in which electrostatic images are produced on an insulative substrate, for example, it may be used to increase the optical density of images produced by thermochromic, photochromic, or free-radical imaging systems. In all such systems, the concept of this invention is applicable through the formation of an image having less than optimum optical density on a shrinkable substrate and subsequently shrinking the substrate to form a reduced-size image having a substantially improved optical density. As used herein, the term "on a substrate" includes the image being actually formed within the substrate such as in the free-radical imaging process.
While no prior art was found in the field of this invention, image intensification, there have been instances in which images have been reduced or expanded in size. For example, a toy was found in which a heat-shrinkable opaque substrate having a design printed in ink thereon is heated to shrink the image and substrate to provide a distorted smaller image. However, the use of this toy to provide amusement for children did not improve the image density inasmuch as the image density was initially of optimum density and no increased optical density resulted from the shrinkage. In another relatively remote art, margin justification of typewritten matter or the like, U.S. Pat No. 1,992,017 proposes to print or type on corrugated paper strips which are then manually stretched or contracted to effect justification of the ends of the lines. Again in another quite different field, reproduction of copy to be transferred to a rubber blanket to be used in letter press and offset printng, U.S. Pat. No. 3,301,127 proposes to compensate for distortion occurring when the blanket is curved around the press cylinder, by distorting the image produced on the flat blanket. However, in none of the above-cited prior procedures was any improvement in optical density of an image contemplated.